FIG. 27 schematically shows the arrangement of a color copying machine as a conventional image forming apparatus, in which image forming units Ya to Yd (image forming means) for four colors are juxtaposed along the convey direction of a paper sheet P (transfer medium).
The image forming units Ya to Yd respectively comprise photosensitive drums 1a to 1d serving as image carriers. A conveyor belt T nearly horizontally extends below the photosensitive drums 1a to 1d. Charging devices 2a to 2d, exposure devices 3a to 3d, developing devices 4a to 4d, transfer devices 5a to 5d, cleaning devices 6a to 6d, and charge removing devices 7a to 7d are disposed around the corresponding photosensitive drums along the rotation direction of the drum. Note that the transfer devices 5a to 5d are disposed on the back surface side of the conveyor belt T.
The image forming unit Ya which is located on the most upstream side along the convey direction of the paper sheet P outputs a yellow image, the second image forming unit Yb a magenta image, the third image forming unit Yc a cyan image, and the last image forming unit Yd a black image.
When a color image is formed on a paper sheet P by the color copying machine with the above-mentioned arrangement, the paper sheet P is conveyed by the conveyor belt T, and image forming processes are done in the order of charging, exposure, development, transfer, cleaning, and charge removal for the surfaces of the photosensitive drums 1a to 1d in the image forming units Ya to Yd. Color developing agent images formed by the individual image forming units Ya to Yd are transferred onto the paper sheet P to overlap each other. The paper sheet P on which the color developing agent images are transferred to overlap each other passes through a fixing device (not shown) which neighbors the fourth image forming unit Yd on its downstream side, and the color developing agent images are melted and fixed on the paper sheet P there, thus forming a color image on the paper sheet P.
However, the conventional color copying machine is constructed by serially disposing four image forming units, each of which constructs a so-called monochrome copying machine for forming a monochrome image, along the convey direction of the paper sheet P.
More specifically, the cleaning devices 6a to 6c except for the cleaning device 6d of the fourth image forming unit Yd, and developing devices 4b to 4d except for the developing device 4a of the first image forming unit Ya are interposed between the neighboring photosensitive drums 1a to 1d. Since the cleaning devices 6a to 6c and developing devices 4b to 4d have predetermined volumes, minimum required spaces must be assured between the neighboring photosensitive drums to attach the devices 4 and 6 between the neighboring photosensitive drums 1a to 1d and to adjust the attachment positions of the devices 4 and 6.
Hence, in the aforementioned conventional color copying machine, given spaces are required between the neighboring photosensitive drums 1a to 1d, and the size of the entire apparatus along the line-up direction of the photosensitive drums 1a to 1d (longitudinal direction), i.e., the convey direction of the paper sheet P, becomes large.
In addition, in the color copying machine, since the individual color image forming units Ya to Yd are used nonuniformly, they must independently undergo exchange, adjustment, and maintenance. For this purpose, a structure that can insert/remove each unit into/from the copying machine independently is required, thus disturbing a size reduction of the apparatus.
In order to attain a size reduction of the apparatus, the photosensitive performance of the photosensitive drums 1a to 1d must be improved. When the process speed is increased to process a large number of paper sheets P within a short period of time in consideration of the photosensitive performance, the exposure-development distance becomes insufficient, and a size reduction is limited in consideration of the sheet convey portion. For example, the exposure-development processing time normally requires 0.15 sec, and it is impossible to reduce the diameter of the photosensitive drum to be smaller than the current diameter.
As a consequence, in order to shorten the size of a color copying machine of this type along the longitudinal direction, it is effective to reduce the pitch between the neighboring photosensitive drums 1a to 1d. For this purpose, size reductions of the developing devices 4b to 4d and cleaning devices 6a to 6c interposed between the neighboring photosensitive drums 1a to 1d need only be promoted.
In the conventional color copying machine, each of the image forming units Ya to Yd has many constituent devices (the photosensitive drum 1, charging device 2, exposure device 3, developing device 4, transfer device 5, cleaning device, and charge removing device 7 will be generally referred to as constituent devices hereinafter), and four sets of these constituent devices must be equipped. For this reason, it is troublesome to manufacture and manage the devices that make up each unit, thus adversely influencing the manufacturing cost. In the image forming units Ya to Yd, since the constituent devices arranged around their photosensitive drums 1a to 1d must be independently and accurately aligned with respect to the photosensitive drums, many manufacturing processes are required, and assembly of the copying machine requires much labor.